Ozone Flux Research Articles

Ozone uptake at night is more damaging to plants than equivalent day-time flux

Main conclusionPlants exposed to equivalent ozone fluxes administered during day-time versus night-time exhibited greater losses in biomass at night and this finding is attributed to night-time depletion of cell wall-localised ascorbate.The present study employed Lactuca sativa and its closest wild relative, L. serriola, to explore the relative sensitivity of plants to ozone-induced oxidative stress during day-time versus night-time. By controlling atmospheric ozone concentration and measuring stomatal conductance, equivalent ozone uptake into leaves was engineered during day and night, and consequences on productivity and net CO2 assimilation rate were determined. Biomass losses attributable to ozone were significantly greater when an equivalent dose of ozone was taken-up by foliage at night compared to the day. Linkages between ozone impacts and ascorbic acid (AA) content, redox status and cellular compartmentation were probed in both species. Leaf AA pools were depleted by exposure of plants to darkness, and then AA levels in the apoplast and symplast were monitored on subsequent transfer of plants to the light. Apoplast AA appeared to be more affected by light–dark transition than the symplast pool. Moreover, equivalent ozone fluxes administered to leaves with contrasting AA levels resulted in contrasting effects on the light-saturated rate of CO2 assimilation (Asat) in both species. Once apoplast AA content recovered to pre-treatment levels, the same ozone flux resulted in no impacts on Asat. The results of the present investigation reveal that plants are significantly more sensitive to equivalent ozone fluxes taken-up at night compared with those during the day and were consistent with diel shifts in apoplast AA content and/or redox status. Furthermore, findings suggest that some thought should be given to weighing regional models of ozone impacts for extraordinary night-time ozone impacts.

Ozone flux-effect relationship for early and late sown Indian wheat cultivars: Growth, biomass, and yield

• Sensitivity of early and late sown Indian wheat cultivars to O 3 were evaluated. • O 3 flux-effect relationships were investigated for AGB and yield in wheat plants. • Higher accumulation of POD 6 SPEC was found in early sown than late sown cultivar. • Higher longevity, g max and lower critical level made early sown cultivars more sensitive. • The grain yield loss under EO 3 was 30.5 % in early and 23 % in late sown cultivars. Precise ozone (O 3 ) flux models remain helpful to identify the sensitivity of crops and their cultivars under high O 3 levels by assessing biomass and yield losses under natural climate and future O 3 levels. In this study, we have evaluated the effects of O 3 on growth, biomass, and yield of four Indian wheat cultivars by exposure-linked as well as through flux-effect relationship-based analysis using two years of empirical data. Early and late sown cultivars were exposed to ambient O 3 (AO 3 ) and elevated O 3 i.e., ambient+20 ppb (EO 3 ). The POD 6 SPEC (species-specific phytotoxic O 3 dose above a threshold of 6 nmol m −2 PLA s -1 ) on flag leaves for each cultivar and treatment was derived by using DO 3 SE (Deposition of Ozone for Stomatal Exchange) model. Above-ground biomass was reduced more in early sown cultivars (26.1 %) than late sown (21.3 %) because of the larger negative impacts on plant growth under EO 3 treatment. Loss of grain yield under EO 3 exposure was high in early sown cultivars (30.5 %) compared to late sown (23 %) due to high stomatal O 3 uptake during the O 3 exposure period. Based on model outputs, early sown cultivars showed higher accumulated POD 6 SPEC (1.6–3.75 mmol O 3 m −2 ) compared to late sown cultivar (0.65 to 1.97 mmol m −2 ). A negative linear relationship was obtained in above-ground biomass and grain yield concerning POD 6 SPEC. The relationship showed that 0.284 mmol O 3 m −2 accumulated POD 6 SPEC is responsible for a 5% reduction of grain yield in early sown cultivars, while 0.393 mmol O 3 m -2 accumulation is required for a 5% reduction in yield of late sown cultivars. The results also suggest that O 3 flux-effect relationship could be a valuable tool for assessing and predicting the risk of O 3 on wheat cultivars in India and for crop productivity models for predicting climate change impacts.

Novel ozone flux metrics incorporating the detoxification process in the apoplast: An application to Chinese winter wheat

A modified Ball-Berry-Leuning model of stomatal conductance was applied to data from fully open-air ozone (O3)-enrichment experiments with winter wheat (Triticum aestivum L.). The O3 fluxes reaching both surface of cell wall (Fcw) and plasmalemma (Fpl) were estimated considering apoplastic ascorbate, a major scavenger of O3. The difference (D) between Fcw and Fpl was defined as detoxification capacity of O3 by reaction with ascorbate in the leaf apoplast (ASCapo). The accumulated stomatal O3 flux above D nmol O3 m−2 s−1 (AFstD) and the accumulated Fpl (AFpl) were calculated over the optimal integration period covering the whole reproductive development of wheat, and used to derive O3AFstD yield-response relationships in comparison with PODY (phytotoxic O3 dose above a threshold of Y nmol m−2 s−1) and AOT40 (accumulated O3 dose over a threshold of 40 ppb). There was a good agreement between the observed and modeled values of ASCapo and stomatal conductance. AFstD and AFpl performed better than PODY and AOT40 in terms of R2 and intercept. However, the AFstD metric was more suitable for assessing grain yield loss due to lower sensitivity of the regression slope to variations in the input parameters, compared with AFpl. The average critical level (CL) of four cultivars for 5% grain-yield reduction was 1.53 mmol m−2 using POD6 and 2.81 mmol m−2 using AFstD, with the latter being well above the POD6-derived value for European cultivars (1.3 mmol m−2). The minimum hourly averaged O3 concentration contributed to CLs was below 20 ppb according to AFstD, a value that is lower than that suggested by POD6 (≈27 ppb). O3 flux-response relationships and CLs on the basis of quantified detoxification capacity shall facilitate the understanding of the different degrees of susceptibility to O3 among species or cultivars, and improve the assessments of O3 impacts on plants.

The response of Pinus species to ozone uptake in different climate regions of Europe

Abstract This study is focused on the research of selected Pinus species exposed to high ozone concentrations in the mountain environment. We noticed different values of modelled ozone doses (MOD) up-taken by Mountain pine (Pinus mugo Turra) in the High Tatra Mts (SK–HTMts) and Swiss stone pine (Pinus cembra L.) in the Alpes-Mercantour (FR–AlpMar) during the growing season 2019. The MOD values were obtained by multiplicative DO3SE model, while we also tested a new approach based on modification of input ozone data. The MOD values were obtained by multiplicative DO3SE model, while we also tested a new approach based on modification of input ozone data. Testing has shown that ozone input based on passive sampling may be used in MOD modelling for sites situated in the subalpine zone where the operation of active monitors is limited.. Presented results confirmed the assumption regarding stomatal ozone flux reduction due to the occurrence of soil drought in hot and dry summer weather typical for the Mediterranean climate region. Despite the limitation of stomatal flux, foliar ozone specific injury on two years needles of P. cembra was substantially higher in comparison to the incidence of ozone injury symptoms observed on two years needles of P. mugo in SK–HTMts. It may suggest low phytotoxicity of given MOD or efficient resistance of P. mugo against oxidative stress. In addition, the visible injury index (VINX) covering the broad effect of biotic and abiotic harmful agents was appraised on P. mugo. Percentage of affected surface indicated moderate deterioration of needle injury at the end of the growing season, particularly due to traces of mechanical damage.

Ozone critical levels for (semi-)natural vegetation dominated by perennial grassland species.

New critical levels for ozone based on accumulated flux through stomata (phytotoxic ozone dose, POD), for temperate perennial grassland (semi-)natural vegetation, have been agreed for use within the Convention on Long-Range Transboundary Air Pollution. These were based on data from several experiments conducted under naturally fluctuating environmental conditions that were combined and analysed to give linear dose-response relationships. Dose-response functions and flux-based critical levels were derived based on biomass and flower number. These parameters showed a statistically significant decline with increasing accumulated stomatal ozone flux. The functions and critical levels derived are based on sensitive species and can be used for risk assessments of the damaging effect of ozone on temperate vegetation communities dominated by perennial grassland species. The critical level based on flower number was lower than that for biomass, representing the greater sensitivity of flower number to ozone pollution.

High nitrogen addition decreases the ozone flux by reducing the maximum stomatal conductance in poplar saplings.

Ground-level ozone (O3) and nitrogen (N) deposition are major environmental pollutants, often occurring concurrently. Ozone exposure- and flux-response relationships for tree biomass are used for regional O3 risk assessment. In order to investigate whether soil N addition affects stomatal O3 uptake of poplar, poplar saplings were exposed to treatment combinations of five O3 levels and four N addition levels. High N addition treatment reduced the accumulated stomatal O3 uptake in the leaf due to reduced maximum stomatal conductance (gs). Nitrogen addition also significantly reduced the steady-state light-saturated gs in August and September. Elevated O3 significantly reduced and N addition increased total plant biomass; however, there were no significant O3×N interactions. The slopes of biomass-based O3 exposure- and flux-response relationships did not differ significantly among N treatments. The critical levels for a 5% biomass reduction were estimated at 15.4ppmh and 17.1mmol O3 m-2 projected leaf area (PLA) for Accumulated O3 exposure Over an hourly Threshold of 40ppb (AOT40) and Phytotoxic Ozone Dose above a threshold 1nmol O3 m-2 PLA s-1 (POD1). These results can facilitate the evaluations of O3 effect on the carbon-sink capacity and productivity of forest.

On the changes in surface ozone over the twenty-first century: sensitivity to changes in surface temperature and chemical mechanisms.

In this study, we show using a state-of-the-art Earth system model, UKESM1, that emissions and climate scenario depending, there could be large changes in surface ozone by the end of the twenty-first century, with unprecedentedly large increases over South and East Asia. We also show that statistical modelling of the trends in future ozone works well in reproducing the model output between 1900 and 2050. However, beyond 2050, and especially under large climate change scenarios, the statistical model results are in poorer agreement with the fully interactive Earth system model output. This suggests that additional processes occurring in the Earth system model such as changes in the production of ozone at higher temperatures or changes in the influx of ozone from the stratosphere, which are not captured by the statistical model, have a first order impact on the evolution of surface ozone over the twenty-first century. We show in a series of idealized box model simulations, with two different chemical schemes, that changes in temperature lead to diverging responses between the schemes. This points at the chemical mechanisms as being a source of uncertainty in the response of ozone to changes in temperature, and so climate, in the future. This underscores the need for more work to be performed to better understand the response of ozone to changes in temperature and constrain how well this relationship is simulated in models. This article is part of a discussion meeting issue 'Air quality, past present and future'.

The impact of drought on total ozone flux in a mountain Norway spruce forest

In order to understand the impact of summer drought on dry deposition of tropospheric ozone (O<sub>3</sub>), we compared severe and mild drought periods of summer 2018 in a mountain Norway spruce forest at Bílý Kříž, Beskydy Mts. An eddy covariance technique was applied to measure diurnal courses of the ecosystem O<sub>3</sub> and CO<sub>2</sub> fluxes. Low O<sub>3</sub> deposition was recorded in the morning and evening, while the highest CO<sub>2</sub> and O<sub><sup>3</sup></sub> fluxes were recorded during the central hours of the day. Total O<sub>3</sub> deposition during severe drought (soil humidity 13%) was significantly higher than the deposition during the mild drought period (soil humidity 19%). Our data indicate that high vapour pressure deficit and low soil humidity during severe drought led to the stomatal closure, while non-stomatal O<sub>3</sub> deposition, associated with chemical reactions of O<sub>3</sub> with NO and volatile organic compounds, are responsible for higher total O<sub>3</sub> deposition during the severe drought period. Therefore, we assume that under severe drought stomatal O<sub>3</sub> uptake decreases but non-stomatal depositions to forest ecosystems substantially increase.

Comparison of Ozone Fluxes over a Maize Field Measured with Gradient Methods and the Eddy Covariance Technique

Ozone (O3) fluxes were measured over a maize field using the eddy covariance (EC) technique and gradient methods. The main objective was to evaluate the performance of the gradient methods for measuring the O3 flux by comparing them with the EC O3 flux. In this study, turbulent exchange coefficients (K) calculated with three methods were compared. These methods were the aerodynamic gradient (AG) method (in which K is calculated by using wind speed and temperature gradients), the aerodynamic gradient combined with EC (AGEC) method, in which the friction velocity and other variables are based on EC measurements, and the modified Bowen ratio using the EC sensible heat flux and temperature gradient (MBR) method. Meanwhile, the effects of the measurement and calculation methods of the O3 concentration gradient were analyzed. The results showed that: (1) on average, the transfer coefficient computed by the MBR method was 40% lower, and the coefficient determined with the AG method was 25% higher, than that determined with the AGEC method. (2) The gradient method’s O3 fluxes with the MBR, AGEC, and AG methods were 30.4% lower, 11.7% higher, and 45.6% higher than the EC O3 flux, respectively. (3) The effect of asynchronous O3 concentration measurements on the O3 gradient must be eliminated when using one analyzer to cyclically measure two-level O3 concentrations. The accuracy of gradient methods for O3 flux is related to the exchange coefficient calculation method, and its precision mainly depends on the quality of the O3 gradient.

Influences of oceanic ozone deposition on tropospheric photochemistry

Abstract. The deposition of ozone to seawater is an important ozone sink. Despite constituting as much as a third of the total ozone deposition, it receives significantly less attention than the deposition to terrestrial ecosystems. Models have typically calculated the deposition rate based on a resistance-in-series model with a uniform waterside resistance. This leads to models having an essentially uniform deposition velocity of approximately 0.05 cm s−1 to seawater, which is significantly higher than the limited observational dataset. Following from Luhar et al. (2018) we include a representation of the oceanic deposition of ozone in the GEOS-Chem model of atmospheric chemistry and transport based on its reaction with sea-surface iodide. The updated scheme halves the calculated annual area-weighted mean deposition velocity to water from 0.0464 cm s−1 (25th and 75th percentiles of 0.0461 cm s−1 and 0.0471 cm s−1 respectively) to 0.0231 cm s−1 (25th and 75th percentiles of 0.0121 cm s−1 and 0.0303 cm s−1 respectively). The calculated ozone deposition velocity varies from 0.009 cm s−1 in polar waters to 0.040 cm s−1 at the tropics. This improves comparisons to observations. The variability is driven mainly by the temperature-dependent rate constant for the reaction between iodide and ozone, the temperature dependence of the solubility, and variations in the ocean iodide concentration. The calculated annual deposition flux of ozone to the ocean is reduced from 222 to 122 Tg yr−1, and overall deposition of ozone to all surface types reduces from 862 to 758 Tg yr−1. Tropospheric ozone burdens and global mean OH increase from 324 to 328 Tg, and from 1.17×106 to 1.18×106 molec.cm-3, respectively. A total of 34 % of surface grid boxes experience a 10 % or greater increase in ozone concentration. Comparisons between observations of surface ozone and the model are improved with the new parameterization notably around the Southern Ocean. Process-level representation of oceanic deposition of ozone thus appears essential for representing the concentration of surface ozone over the planet.

Investigation of Transport and Transformation of Tropospheric Ozone in Terrestrial Ecosystems of the Coastal Zone of Lake Baikal

Forest ecosystems play an important role in the process of removing trace gases from the atmosphere. The purpose of this work is the experimental study of the flux and rate of the dry deposition of ozone on different types of vegetation in the Baikal region. Based on the eddy covariance method and the flux gradient methods (including the aerodynamic gradient method (AGM), the modified Bowen method (MBR) and modified gradient method (MGM)) and with experimental data, the quantitative estimates of the fluxes and dry deposition velocity of ozone on the underlying surface were obtained for different environments (grasslands, forest). The average values of the dry deposition velocity of ozone (Vd) were equal to 0.37 cm/s at night (0–3 h) and 0.91 cm/s during daytime hours (12–18 h). The ozone flux (F) was 0.24 μg m–2 s–1 at night and 0.72 μg m–2 s–1 in the afternoon. The quantitative estimates of Vd and F on vegetation and in the forest show a higher absorptive capacity of forests compared to soil vegetation.

Sensitivity of Meridional Mean Circulation to the Impact of Orographic Waves at Different Phases of Quasi-Biennial Oscillations in a Numerical Model of the Middle Atmosphere

A parameterization of dynamical and thermal effects of orographic gravity waves (OGWs) has been included into the numerical model of the general atmospheric circulation of the middle and upper atmosphere. The sensitivity of mean meridional circulation to the impact of stationary OGW during the years with different quasi-biennial oscillation (QBO) phases at altitudes up to 100 km in January has been studied. The respective changes in vertical mean ozone fluxes in the atmosphere have been observed. The account of stationary OGW effects leads to the changes up to 40% in the meridional velocity and the associated ozone fluxes in the stratosphere. In the winters with the easterly and westerly QBO phases, the differences between the maximum meridional velocities in the middle atmosphere at the middle and high latitudes of the northern hemisphere can reach 60%.

Effects of Environmental Factors on Ozone Flux over a Wheat Field Modeled with an Artificial Neural Network

Ozone (O3) flux-based indices are considered better than O3 concentration-based indices in assessing the effects of ground O3 on ecosystem and crop yields. However, O3 flux (Fo) measurements are often lacking due to technical reasons and environmental conditions. This hampers the calculation of flux-based indices. In this paper, an artificial neural network (ANN) method was attempted to simulate the relationships between Fo and environmental factors measured over a wheat field in Yucheng, China. The results show that the ANN-modeled Fo values were in good agreement with the measured Fo values. The R2 of an ANN model with 6 routine independent environmental variables exceeded 0.8 for training datasets, and the RMSE and MAE were 3.074 nmol·m−2·s and 2.276 nmol·m−2·s for test dataset, respectively. CO2 flux and water vapor flux have strong correlations with Fo and could improve the fitness of ANN models. Besides the combinations of included variables and selection of training data, the number of neurons is also a source of uncertainties in an ANN model. The fitness of the modeled Fo was sensitive to the neuron number when it ranged from 1 to 10. The ANN model consists of complex arithmetic expressions between Fo and independent variables, and the response analysis shows that the model can reflect their basic physical relationships and importance. O3 concentration, global radiation, and wind speed are the important factors affecting O3 deposition. ANN methods exhibit significant value for filling the gaps of Fo measured with micrometeorological methods.

Ozone and particle fluxes in a Mediterranean forest predicted by the AIRTREE model

Mediterranean forests are among the most threatened ecosystems by the concurrent effects of climate change and atmospheric pollution. In this work we parameterized the AIRTREE multi-layer model to predict CO2, water, ozone, and fine particles exchanges between leaves and the atmosphere. AIRTREE consists of four different modules: (1) a canopy environmental module determines the leaf temperature and radiative fluxes at different levels from above to the bottom of the canopy; (2) a hydrological module predicts soil water flow and water availability to the plant's photosynthetic apparatus; (3) a photosynthesis module estimates the net photosynthesis and stomatal conductance, and (4) a deposition module estimates ozone and PM deposition sinks as a function of the resistances to gas diffusion in the atmosphere, and within the canopy and leaf boundary layer. We describe the AIRTREE model framework, accuracy and sensitivity by comparing modeling results against long-term continuous Eddy Covariance measurements of ozone, water, and CO2 fluxes in a Mediterranean Holm oak forest, and we discuss potential application of AIRTREE for ozone-risk assessment in view of availability of a large observational database from ecosystems distributed worldwide.

Ozone flux and ozone deposition in a mountain spruce forest are modulated by sky conditions

In order to understand the main driving factors of ozone (O3) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and clear sky) modulate O3 flux in forest ecosystems via stomatal regulation. The hypothesis is based on the fact that complex microclimate conditions under cloudy sky usually stimulate stomatal conductance.O3 fluxes were inferred from a concentration gradient in a mountainous Norway spruce forest in the Czech Republic (Central Europe) for years 2012–2016 and measured directly by eddy-covariance during the summer of 2017. Daily and seasonal O3 depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions.The data show unequivocally that more O3 is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O3 flux. Though there are other mechanisms and pathways involved in the transport of O3 to the plant-soil system, the highest O3 deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O3 ecosystem fluxes were observed under clear sky conditions despite the highest O3 concentrations at this time. These findings suggest that forests growing at sites where conditions are predominantly cloudy are expected to deposit higher extent of O3 than less-cloudy forests being thus more threatened by phytotoxic O3.

Prediction of ozone effects on net ecosystem production of Norway spruce forest

Future ground-level concentrations of phytotoxic ozone are projected to grow in the Northern Hemisphere, at a rate depending on emission scenarios. We explored the likely changes in net ecosystem production (NEP) due to the increasing concentration of tropospheric ozone by applying a Generalized Additive Mixed Model based on measurements of ozone concentration ([O3]) and stomatal ozone flux (FsO3), at a mountainous Norway spruce forest in the Czech Republic, Central Europe. A dataset covering the growing period (May-August 2009) was examined in this case study. A predictive model based on FsO3 was found to be marginally more accurate than a model using [O3] alone for prediction of the course of NEP when compared to NEP measured by the eddy covariance technique. Both higher [O3] and FsO3 were found to reduce NEP. NEP simulated at low, pre-industrial FsO3 (0.5 nmol m-2 s-1) was higher by 24.8% as compared to NEP assessed at current rates of FsO3 (8.32 nmol m-2 s-1). However, NEP simulated at high FsO3 (17 nmol m-2 s-1), likely in the future, was reduced by 14.1% as compared to NEP values at current FsO3. The interaction between environmental factors and stomatal conductance is discussed in this paper.

Characterization of ozone deposition to a mixed oak–hornbeam forest – flux measurements at five levels above and inside the canopy and their interactions with nitric oxide

Abstract. A 1-month field campaign of ozone (O3) flux measurements along a five-level vertical profile above, inside and below the canopy was run in a mature broadleaf forest of the Po Valley, northern Italy. The study aimed to characterize O3 flux dynamics and their interactions with nitrogen oxides (NOx) fluxes from the forest soil and the atmosphere above the canopy. Ozone fluxes measured at the levels above the canopy were in good agreement, thus confirming the validity of the constant flux hypothesis, while below-canopy O3 fluxes were lower than above. However, at the upper canopy edge O3 fluxes were surprisingly higher than above during the morning hours. This was attributed to a chemical O3 sink due to a reaction with the nitric oxide (NO) emitted from soil and deposited from the atmosphere, thus converging at the top of the canopy. Moreover, this mechanism was favored by the morning coupling between the forest and the atmosphere, while in the afternoon the fluxes at the upper canopy edge became similar to those of the levels above as a consequence of the in-canopy stratification. Nearly 80 % of the O3 deposited to the forest ecosystem was removed by the canopy by stomatal deposition, dry deposition on physical surfaces and by ambient chemistry reactions (33.3 % by the upper canopy layer and 46.3 % by the lower canopy layer). Only a minor part of O3 was removed by the understorey vegetation and the soil surface (2 %), while the remaining 18.2 % was consumed by chemical reaction with NO emitted from soil. The collected data could be used to improve the O3 risk assessment for forests and to test the predicting capability of O3 deposition models. Moreover, these data could help multilayer canopy models to separate the influence of ambient chemistry vs. O3 dry deposition on the observed fluxes.

A Site-Specific Analysis of the Implications of a Changing Ozone Profile and Climate for Stomatal Ozone Fluxes in Europe

In this study, we used eight sites from across Europe to investigate the implications of a future climate (2 °C warmer and 20% drier) and a changing ozone profile (increased background concentrations and reduced peaks) on stomatal ozone fluxes of three widely occurring plant species. A changing ozone profile with small increases in background ozone concentrations over the course of a growing season could have significant impacts on the annual accumulated stomatal ozone uptake, even if peak concentrations of ozone are reduced. Predicted increases in stomatal ozone uptake showed a strong relationship with latitude and were larger at sites from northern and mid-Europe than those from southern Europe. At the sites from central and northern regions of Europe, including the UK and Sweden, climatic conditions were highly conducive to stomatal ozone uptake by vegetation during the summer months and therefore an increase in daily mean ozone concentration of 3–16% during this time of year (from increased background concentrations, reduced peaks) would have a large impact on stomatal ozone uptake. In contrast, during spring and autumn, the climatic conditions can limit ozone uptake for many species. Although small increases in ozone concentration during these seasons could cause a modest increase in ozone uptake, for those species that are active at low temperatures, a 2 °C increase in temperature would increase stomatal ozone uptake even in the absence of further increases in ozone concentration. Predicted changes in climate could alter ozone uptake even with no change in ozone profile. For some southern regions of Europe, where temperatures are close to or above optimum for stomatal opening, an increase in temperature of 2 °C could limit stomatal ozone uptake by enhancing stomatal closure during the summer months, whereas during the spring, when many plants are actively growing, a small increase in temperature would increase stomatal ozone uptake.

Response of Arctic ozone to sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) are the main source of intra-seasonal and interannual variability in the extratropical stratosphere. The profound alterations to the stratospheric circulation that accompany such events produce rapid changes in the atmospheric composition. The goal of this study is to deepen our understanding of the dynamics that control changes of Arctic ozone during the life cycle of SSWs, providing a quantitative analysis of advective transport and mixing. We use output from four ensemble members (60 years each) of the Whole Atmospheric Community Climate Model version 4 performed for the Chemistry Climate Model Initiative and also use reanalysis and satellite data for validation purposes. The composite evolution of ozone displays positive mixing ratio anomalies of up to 0.5–0.6 ppmv above 550 K (∼ 50 hPa) around the central warming date and negative anomalies below (−0.2 to −0.3 ppmv), consistently in observations, reanalysis, and the model. Our analysis shows a clear temporal offset between ozone eddy transport and diffusive ozone fluxes. The initial changes in ozone are mainly driven by isentropic eddy fluxes linked to enhanced wave drag responsible for the SSW. The recovery of climatological values in the aftermath of SSWs is slower in the lower than in the upper stratosphere and is driven by the competing effects of cross-isentropic motions (which work towards the recovery) and isentropic irreversible mixing (which delays the recovery). These features are enhanced in strength and duration during sufficiently deep SSWs, particularly those followed by polar-night jet oscillation (PJO) events. It is found that SSW-induced ozone concentration anomalies below 600 K (∼ 40 hPa), as well as total column estimates, persist around 1 month longer in PJO than in non-PJO warmings.

Effect of helical structure on ozone mass transfer in a hollow fiber membrane contactor

The aims of this work were to investigate the ozone mass transfer in a helically wounded hollow fiber membrane contactor in dependence on operating conditions and to study the effect of the helical membrane structure on the ozonation performance compared to linear fiber arrangement. The role of the mass transfer resistances and the fiber geometry were investigated by means of a mathematical model. The ozone flux was found to increase with increasing gas pressure, ozone gas concentration, gas and liquid velocities. In contrast, for a high ozone dosage low liquid velocities should be preferred. The helically wounded structure of the hollow fibers was found to enhance the ozone flux up to factor 7 comparing to linear fibers due to arising secondary flows inside the fiber lumen and an orthogonal flow at fiber shell. Especially at high Reynolds the mass transfer resistance of the liquid boundary layer in a helical fiber membrane contactor is significantly reduced, so that the resistance of the gas boundary layer gains importance. The mass transfer resistance of the membrane remains low comparing to the overall mass transfer resistance. The possible application fields for ozonation purposes using helical fiber membrane contactor and economic benefits compared to linear fiber membrane contactors could be derived based on the gained ozonation results.